Chair back



FIG. 1 is a front, right perspective view of a chair back showing our new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is a front elevational view thereof; and,

FIG. 7 is a bottom plan view thereof. 

The ornamental design for a chair back, as shown and described. 